It is recognized more and more that the expression of many prokaryotic and eukaryotic genes is regulated at the level of elongation by premature termination. Various oncogenes and retroviral genes show regulation at this level. Furthermore, termination at the 3' end of a gene can influence the expression of other genes located downstream (promoter occlusion). The well-characterized ribosomal genes of Xenopus laevis are an attractive model system in which to study those processes because they show phenomena of both premature termination and of promoter occlusion. Furthermore, a 9 base-pair DNA signal that directs RNA 3' end formation and transcription termination has been identified. The same signal is also involved in processive transcription elongation. This application proposes to identify and purify proteins involved in the processes of transcription elongation, RNA 3' end formation and termination by RNA polymerase I. An S-100 cell extract will be used that is active in transcription initiation and termination and, therefore, contains all those activities. This extract will be fractionated and the sought activities will be identified either by functional assays or by binding assays to postulated target sequences.